The shrink index API allows you to shrink an existing index into a new index
with fewer primary shards. The requested number of primary shards in the target index
must be a factor of the number of shards in the source index. For example an index with
8 primary shards can be shrunk into 4, 2 or 1 primary shards or an index
with 15 primary shards can be shrunk into 5, 3 or 1. If the number
of shards in the index is a prime number it can only be shrunk into a single
primary shard. Before shrinking, a (primary or replica) copy of every shard
in the index must be present on the same node.

Shrinking works as follows:

First, it creates a new target index with the same definition as the source
index, but with a smaller number of primary shards.

Then it hard-links segments from the source index into the target index. (If
the file system doesn’t support hard-linking, then all segments are copied
into the new index, which is a much more time consuming process.)

Finally, it recovers the target index as though it were a closed index which
had just been re-opened.

Forces the relocation of a copy of each shard to the node with name
shrink_node_name. See Shard Allocation Filtering for more options.

Prevents write operations to this index while still allowing metadata
changes like deleting the index.

It can take a while to relocate the source index. Progress can be tracked
with the _cat recovery API, or the cluster health API can be used to wait until all shards have relocated
with the wait_for_no_relocating_shards parameter.

To shrink my_source_index into a new index called my_target_index, issue
the following request:

POST my_source_index/_shrink/my_target_index

The above request returns immediately once the target index has been added to
the cluster state — it doesn’t wait for the shrink operation to start.

Indices can only be shrunk if they satisfy the following requirements:

the target index must not exist

The index must have more primary shards than the target index.

The number of primary shards in the target index must be a factor of the
number of primary shards in the source index. The source index must have
more primary shards than the target index.

The index must not contain more than 2,147,483,519 documents in total
across all shards that will be shrunk into a single shard on the target index
as this is the maximum number of docs that can fit into a single shard.

The node handling the shrink process must have sufficient free disk space to
accommodate a second copy of the existing index.

The _shrink API is similar to the create index API
and accepts settings and aliases parameters for the target index:

The shrink process can be monitored with the _cat recovery API, or the cluster health API can be used to wait
until all primary shards have been allocated by setting the wait_for_status
parameter to yellow.

The _shrink API returns as soon as the target index has been added to the
cluster state, before any shards have been allocated. At this point, all
shards are in the state unassigned. If, for any reason, the target index
can’t be allocated on the shrink node, its primary shard will remain
unassigned until it can be allocated on that node.

Once the primary shard is allocated, it moves to state initializing, and the
shrink process begins. When the shrink operation completes, the shard will
become active. At that point, Elasticsearch will try to allocate any
replicas and may decide to relocate the primary shard to another node.